Thanks Jarcec! I left a comment (no action required from your side) on Reviewboard that "false" seems to be the right value to put for existing rows. Perhaps, Gang Tim Liu or [~namitjain], who were involved with
HIVE-3649 can confirm.
I like the new approach of adding the new column as Nullable, updating the existing null values and then setting the column to not-null. The only other approach I can think of is to use a default value (of false) but that seems more risky in the long run.